Method for glazing ceramic articles obtained by slip casting

ABSTRACT

Method of glazing ceramic articles, consisting of preparing a glaze compound in liquid suspension, preferably aqueous; feeding the glaze compound in liquid suspension, preferably aqueous, into the forming cavity of a porous mould, for the time required to create a glaze film of desired thickness on the surface of the cavity by absorption of the liquid phase; feeding the slip into the same cavity for the time required to create the wall of the article by absorption of the liquid phase.

TECHNICAL FIELD

[0001] This invention relates to a method for glazing ceramic articles obtained by slip casting within the cavity of a porous mould, and the glaze used in said method.

BACKGROUND ART

[0002] This type of ceramic article is glazed either manually or by appropriate robotized machines, after the piece has been formed and dried.

[0003] In the case of certain ceramic articles, such as sanitary appliances, the automatic glazing machines are unable to completely glaze all the surfaces of the article, it being possible to glaze only the accessible surfaces, i.e. the visible surfaces.

[0004] For example in the case of water closet pans, the rim and the stench trap are currently glazed only on their outer surfaces; likewise it is not possible to glaze the overflow channels of wash-basins and bidets.

[0005] Consequently the final product presents unglazed surfaces where, in the course of time, bacterial growth can occur rendering the article unhygienic. Current glazing techniques involve a separate glazing operation which negatively affects the production time and hence cost.

[0006] Finally, known glazing machines are costly and complicated.

[0007] The object of this invention is to overcome the aforementioned drawbacks of the known art within the context of a rational and reliable solution. The invention attains said object by virtue of the characteristics defined in the claims.

[0008] The invention consists of a method which enables the usual glazing machines to be completely eliminated by glazing the article directly within the forming cavities of the article mould, by injecting into them a glaze compound in liquid suspension, preferably aqueous.

[0009] In detail, the method of the invention comprises the following operative steps:

[0010] preparing a glaze compound in liquid suspension,

[0011] feeding the glaze compound in suspension into the article-forming cavities, for a time sufficient to create a glaze film able to uniformly cover the cavity walls by absorption of the suspension liquid phase;

[0012] feeding the slip into the article-forming cavities for a time sufficient to create the article wall by absorption of the slip liquid phase;

[0013] possibly feeding the glaze compound into those article-forming cavities in which a film of glaze and a layer of slip have already been deposited, for a time sufficient to create a glaze film able to uniformly cover the inner walls of the layer of slip by absorption of the liquid phase.

[0014] The method of the invention can in fact be applied either to the outer and inner surfaces of the article or to only one of the two surfaces, depending on the type of article to be constructed. The method does not require special equipment for its implementation, it using the actual equipment used to form the article.

[0015] Said equipment is simply completed by means for feeding and recirculating the liquid glaze suspension, arranged in parallel with the already existing means for feeding and recirculating the slip.

[0016] The invention also comprises a glaze suitable for use with the method, which glaze must have characteristics not always present in usual glazes. A suitable glaze must present, during firing, the same chemical and physical characteristics as a usual glaze, whereas during forming and drying it must behave in a manner similar to the layer deposited by the slip.

[0017] The co-existence of said characteristics is obtained, according to the invention, by providing in the glaze composition a clay material component which conveniently exceeds 10% and has the function of increasing the plasticity of the glaze and regularizing its shrinkage.

[0018] Said clay material can be an illite, kaolinite, chlorite or montmorillonite clay, or a mixture of said clays. Moreover, said clay material component of said glaze is preferably between 15 and 35%.

[0019] Three possible glaze compositions suitable for use in this method are given below in the form of tables. GLAZE 1 GLAZE 2 GLAZE 3 Raw Materials % % % Sanblend 90 20 Clay UK1 20 Kaolin LPC 20 Feldspar F7 10 9 K Feldspar FS 900 15 23.1 Quartz P4 16 18 13.4 CaSiO₃ Vansil EW20 13.5 16 23 ZnO Zinc Oxide 18 1 Zircobit MO 1 13 Zircosil 5 13.5 Barium Carbonate 6.5 6.5 5.4 Talc SSB 60 1.5 1.5 1.6

[0020] If only the outer surface of the article is to be glazed, the glaze suspension in a liquid phase, preferably water, is injected into the mould cavities before forming the article and the liquid phase is then absorbed to create a film of consolidated glaze, of the desired thickness, on the surface of the mould cavity to form the outer surface of the article.

[0021] When the film of glaze has been created, the slip is cast in the same mould cavity to form the ceramic article wall, to which said glaze film adheres.

[0022] When the article has been removed from the mould, it presents a glazed outer surface.

[0023] When the wall of the article has been formed by casting the slip, the inner surface of the article is glazed by injecting the glaze in liquid suspension, preferably aqueous, into the same mould cavity so that a film of consolidated glaze forms adhering to all inner surfaces of the article. By virtue of the invention, the production of ceramic articles obtained by slip casting is less costly, in that the production plant does not require special glazing machines as glazing is implemented in the actual forming cavities of the mould.

[0024] Moreover from the aforegoing, it is apparent that the articles produced using the glazing method of the invention are uniformly glazed in all their parts, both visible and those hidden from the user's view. In certain applications, such as sanitary appliances, it is hygienically very important that the glazing of the article be complete, as this reduces bacterial growth which would render the product unhygienic.

[0025] To apply the method of the invention it is sufficient, as described, to provide the mould with two feed circuits, one dedicated to the slip and the other dedicated to the liquid glaze suspension. 

1. A method for glazing ceramic articles, characterised by: preparing a glaze compound in liquid suspension, preferably aqueous; feeding the glaze compound in suspension, preferably aqueous, into the forming cavities of a porous mould, for a time necessary to create a glaze film of desired thickness on the surface of the cavity walls by absorption of the liquid phase; feeding the slip into the article-forming cavities for the time necessary to create the article wall by absorption of the liquid phase.
 2. A method as claimed in claim 1, characterised in that, after feeding the slip, further glaze compound in aqueous suspension is fed into the mould cavity already containing the film of glaze and the article wall, to create a further film of glaze by absorption of the suspension aqueous phase.
 3. A method as claimed in claim 1 characterised in that the glaze compound in aqueous suspension is fed under pressure.
 4. A method as claimed in claim 1 characterised in that the glaze composition comprises from 10 to 60% of clay component.
 5. A method as claimed in claim 4 characterised in that said clay component is one or more chosen from the following materials: ball clay, kaolin, illite clay, chlorite clay and montmorillonite clay.
 6. A method as claimed in claim 4 characterised in that the glaze comprises one or more of the following components: Sanblend 90 Clay UK1 Kaolin LPC Feldspar F7 K Feldspar FS 900 Quartz P4 CaSiO₃ Vansil EW20 ZnO Zinc Oxide Zircobit MO Zircosil 5 Barium Carbonate Talc SSB 60
 7. A method as claimed in claim 4 characterised in that the glaze comprises the following components: Sanblend 90 Feldspar F7 K Feldspar FS 900 Quartz P4 CaSiO₃ Vansil EW20 ZnO Zinc Oxide Zircobit MO Barium Carbonate Talc SSB 60
 8. A method as claimed in claim 4 characterised in that the glaze comprises one or more of the following components: Kaolin LPC Feldspar F7 K Feldspar FS 900 Quartz P4 CaSiO₃ Vansil EW20 ZnO Zinc Oxide Zircobit MO Barium Carbonate Talc SSB 60
 9. A method as claimed in claim 4 characterised in that the glaze comprises one or more of the following components: Clay UK1 Feldspar F7 K Feldspar FS 900 Quartz P4 CaSiO₃ Vansil EW20 ZnO Zinc Oxide Zircobit MO Zircosil 5 Barium Carbonate Talc SSB 60
 10. A glaze in aqueous suspension for ceramic articles, characterised by comprising more than 10% of at least one clay component.
 11. A glaze as claimed in claim 10, characterised by comprising from 15 to 35% of said clay component.
 12. A glaze as claimed in claim 10 characterised in that said clay component is one or more chosen from the following materials: ball clay, kaolin, illite clay, chlorite clay and montmorillonite clay.
 13. A glaze as claimed in claim 10 characterised by comprising at least one of the following materials: Quartz P4 CaSiO₃ Vansil EW20 ZnO Zinc Oxide Zircobit MO Zircosil 5 Barium Carbonate Talc SSB 60
 14. A glaze as claimed in claim 10 characterised by comprising at least one of the following materials: Sanblend 90 Feldspar F7 K Feldspar FS 900 Quartz P4 CaSiO₃ Vansil EW20 ZnO Zinc Oxide Zircobit MO Barium Carbonate Talc SSB 60
 15. A method as claimed in claim 10 characterised by comprising at least one of the following materials: Kaolin LPC Feldspar F7 K Feldspar FS 900 Quartz P4 CaSiO₃ Vansil EW20 ZnO Zinc Oxide Zircobit MO Barium Carbonate Talc SSB 60
 16. A method as claimed in claim 10 characterised by comprising at least one of the following materials: Clay UK1 Feldspar F7 K Feldspar FS 900 Quartz P4 CaSiO₃ Vansil EW20 ZnO Zinc Oxide Zircobit MO Zircosil 5 Barium Carbonate Talc SSB 60 